Heat the vegetable oil in a large pan on a low heat
Add the onion, garlic, carrot, celery and courgette. Cook for 8-10 minutes until the onion has softened
Add the chopped tomatoes, vegetarian mince, tomato puree, balsamic vinegar, marmite, vegetable stock cube, red wine stock cube, water and mixed herbs
Stir everything together and bring to a gentle simmer, simmer for 20 minutes
While it is simmering, prepare the potatoes and place into a pan of boiling water. Boil them for 15-20 minutes until soft, then drain them
Pre-heat your oven to 180C Fan/400F/Gas Mark 6
Mash the potatoes up with the milk and butter
Add the lentils to the filling and stir them in. Then taste the filling mixture and add salt and pepper to your preference, then put it in your baking dish
Carefully top the filling with the mash potato. Run a fork all over the top to create texture (this helps to make the top lovely and crispy)
Sprinkle the grated cheese all over the pie
Cook for 20-25 minutes until golden on top and bubbling
Remove from the oven and serve
Fully cooled leftovers can be stored in the fridge for 3 days or the freezer for 3 months